(1) Field of Invention
The present invention relates to gypsum wallboard and more particularly refers to a method for coating the bond liner surface of a paper cover sheet with a starch adhesive composition in a selected pattern, to the product formed thereby, and to gypsum wallboard formed by applying a calcined gypsum slurry to the coated bond liner surface of the paper cover sheet.
(2) Description of the Prior Art
Conventional wallboard manufacture has in the past been based on the theory that the wet and dry bond were one and the same, that the paper-to-core bond was mechanical and crystalline, and that drying calcined the crystalline bond to the point that the bond was essentially destroyed. To prevent this, starch was usually introduced into the core slurry and permitted to migrate to the paper-core interface to protect the gypsum crystals forming the mechanical bond. In contrast to this, it was disclosed in U.S. Ser. No. 491,097 of which this application is a continuation-in-part, that the paper-to-core bond actually proceeds in a two-step fashion, as follows. When the slurry is cast onto the paper cover sheets, and specifically the bond liner thereof, wet or green bonding will occur between the bond liner and the slurry due to hydrogen bonding, unless the paper is treated to prevent such hydrogen bonding. For example, any bond liner sizing tends to reduce such hydrogen bonding, and any film completely coating the liner eliminates the hydrogen bonding altogether. Upon drying the board in conventional high temperature kilns, no matter to what degree wet bonding existed prior to drying, it is all destroyed during drying. Thus, for dry bond to take place, a replacement for the destroyed hydrogen bonds must be introduced. Since the dry bond takes place between the bond liner and the gypsum core, the adhesive must be maintained at the paper-core interface until it cures or sets.
It was generally found that the adhesive must be coated upon the bond liner of the cover sheet in such a manner as to permit and maintain wet bonding of the cover sheet during the casting of the wallboard. This required that the adhesive must be non-film forming until it cures, and must be for that reason applied in a discontinuous manner in order that a substantial area of the bond liner surface remains uncoated. The adhesive must further be one which will not migrate from the paper-core interface during the casting and drying of the wallboard, and yet one which will set or cure during the drying before the wet bond has been completely destroyed. As a result of the interplay of wet bonding and subsequent dry bonding, a wallboard is formed which, when dry, retains the adhesive in a discontinuous or spaced-apart or partially covered pattern at the core-to-paper interface, because of the non-migratory nature of the adhesive.
It was previously found that certain uncooked or raw starches and other adhesives are in fact non-migratory and therefore do not spread to completely cover the surface of the cover sheets, but leave substantial areas uncoated and free of the adhesive. Consequently a wallboard paper-to-core bond is formed which is uniformly free of "peelers" and paper "blows".
As disclosed in the prior application referred to above, it was found that when a slurry of raw starch was coated on a paper cover sheet in a discontinuous pattern and in such a manner that substantial areas of the cover sheet were left uncoated even when dry, it was found that the thus coated cover sheets could be utilized either in the wet or dried stage to form gypsum wallboard by applying an aqueous calcined gypsum slurry which itself did not contain starch to the coated paper cover sheets.
In seeking convenient commercial methods for preparing the starch-coated paper cover sheets, it was found that an excellent product could be produced by means of a roller coater having spaced-apart annular ridges. However, because of the relatively low viscosity of the starch coating composition, it was found difficult to apply sufficient coating material without having the slurry creep over on areas which were desired to be left uncovered.
In preparing starch-coated paper having substantial areas free of starch it has been found that an excellent product can be produced by providing the starch coatings in the form of discrete and definite "striped" patterns or designs wherein the stripes are substantially parallel to each other. It was found that such patterns could be applied to paper with better control and reproduceability by means of a grooved, resilient applicator roll using a direct roll coater. However, in attempting to use a direct roll coater to apply the starch coating compositions disclosed in the above-mentioned application, U.S. Ser. No. 491,097, it was found that the coating material was not completely satisfactory for such application due in part to the properties of the composition, and due particularly to a low Brookfield viscosity of about 1000 cps. It became clear that compositions having higher viscosities and providing "stiffer" coatings were needed to maintain extended, continuous and trouble-free operations with the direct roll coater process. Original efforts to improve the physical properties of the coating composition to obtain a high viscosity were generally unsuccessful. When the proportion of starch was increased to obtain the necessary viscosity, this resulted in a coating of relatively dry character and which could not be utilized for forming a suitable coated paper by means of the direct roll coater. Moreover, an increase in the amount of dispersing agent to obtain a suitable viscosity resulted in the formation of a "gummy" coating which caused additional problems in the coating operation, as for example, extensive water holding capacities which affected both the drying rate of the coating on the paper as well as the imprinting operation, as for example, flowing of the coating on the total sheet which resulted in poor pattern definition and some overcoating of the areas intended to be free of the adhesive coating. Moreover, the use the additional dispersing agent resulted in a prohibitively high material cost.